PL/pgSQL - args passing

Is it possible to pass the NULL value to PL/pgSQL function? More precisely,
I have a function like

CREATE FUNCTION test (text,int4) RETURNS int4 AS '
Begin
...
End;
' LANGUAGE 'plpgsql';

and typical call is

select test('Word',1);

which works fine. But sometimes I need to call this function with
arguments like

select test(NULL,1); or
select test('Word',NULL);

Now such calls lead to
ERROR: typeidTypeRelid: Invalid type - oid = 0

Any ideas?
_________________________________________________________________________
Evgeny Rodichev Sternberg Astronomical Institute
System/Net Admin Moscow State University
email: er(at)sai(dot)msu(dot)su
Phone: 007 (095) 939 2383
Fax: 007 (095) 932 8841 http://www.sai.msu.su/~er